The objective is to characterize the natural history of the organism of Legionnaires' disease. Experiments, divided into three groups, will attempt to: a) Clarify the ecologic niche: We will investigate the morphology of the transition from intracellular to cell-free growth, and the potential of spore-like structures to germinate; b) Elucidate mode of transmission: By pathologic techniques, we will compare the pathology produced in guinea pigs by 3 routes of inoculation: air, water and intraperitoneal; c) Delineate pathology: By light, immunofluorescent and electron microscopy we will describe the pathology of Legionnaires' disease in guinea pigs and humans, including distribution and features of the organisms within lesions. We will attempt to develop a diagnostic test for the acute phase of the illness. Data from the experiments will be used to clarify the taxonomy of the organism.